From the medical point, skin performs a function as a protection film at the outermost part of human body, and performs important functions for preventing loss of body ingredients such as water, electrolyte, and the like, and simultaneously blocking inflow of external harmful substances. Particularly, stratum corneum forms the outermost layer of skin to prevent moisture loss inside of skin from dry external environment, and simultaneously, it holds appropriate moisture and exhibits flexibility and elasticity.
In the stratum corneum of normal skin, 10% or more of moisture is commonly contained to afford elasticity and flexibility. The degree of moisture content decreases from dermis to epidermis. Stratum corneum consists of about 15% water, 70% protein and 15% lipid, while dermis, particularly keratinocyte consists of 70% water, 15% protein and 5% lipid. As the keratinocyte approaches to the surface, moisture content gradually decreases, and finally reaches to 5˜10%. The outermost part of skin, even if the skin is very good, does not contain 20% or more of water. The moisture content of stratum corneum of moist skin is commonly around 15%.
However, skin loses water balance due to various factors including external environments such as aging, change in hormone secretion, wind, temperature and sun, and the like, and physical factors such as wash and shave, and the like, and it easily becomes dry skin with moisture content of 10% or less. Thus, skin loses elasticity and flexibility, and eventually, skin protection function disappears to induce skin fissure, erythema, pruritus, and the like, and more worse, generate skin disease such as psoriasis, atopic dermatitis, and the like.
To solve these problems, it may be ideal to solve causative factors, but since the causes are various and mostly uncontrollable, the second-best solution is being considered. It may include a method of blocking water evaporation of skin cells to prevent decrease in skin moisture content, and a method of minimizing irritation from external environment by forming an artificial moisturization film on the skin. Among them, a method of forming an artificial moisturization film is most simple, and for this, various kinds of skin moisturizers and moisturizing cosmetics are being developed.
However, skin friendly materials that are conventionally used as raw materials of moisturizer and moisturizing cosmetics have various problems. For example, although polyhydric alcohols such as glycerin or sorbitol that are mostly used as moisturizer ingredients exhibit excellent moisturizing effect, they cause serious stickiness to induce displeasure, and they are easily separated on the skin surface due to physical influences and thus long-lasting effects cannot be expected. And, raw materials such as propyleneglycol, 1,3-butyleneglycol may cause side effects to skin such as erythema, when repeatedly used in a large amount. Moreover, natural moisturizing factors (NMF) of Na-lactate, sodium pyrrolidone carboxylic acid (PCA-Na), urea, and the like have strong electrolytic property, and thus, they have problems of damaging emulsion stability of cosmetics or moisturizer, and the like, and the effects are limited.
Recently, there are increasing efforts to form a moisturization film on the skin using biological materials or biomaterials instead of chemical materials. Among them, there are many efforts to use natural extracts, various proteins and decomposition products thereof as moisturizing ingredients, and practically, products containing these ingredients have been developed and are commercially available. As the protein decomposition product, collagen decomposition product, elastin decomposition product, silk decomposition product, and the like are mainly used. However, they do not exhibit constant moisturizing effects, and they should be contained in high concentrations to achieve moisturizing effects because the effects are slight.
Particularly, silk which is protein-based fiber consisting of fibroin and sericin has been mainly used as cloth in the past, but recently, it has been found through the studies that silk protein and its decomposition products, peptides and amino acids have the effects for lowering blood cholesterol concentration, relieving hangovers, and preventing dementia, and thus, it draws attentions as health functional foods and cosmetics. And, it has been disclosed in various patents and papers that silk-derived protein and its decomposition product, silk peptide posses moisturizing effects and thus may be used as cosmetics.
Specifically, Korean Registered Patent No. 10-0428431 discloses use of decomposition product of silk protein by proteinase (low molecular weight silk protein), Korean Registered Patent No. 10-0652122 discloses use of hydrolysis product of sericin, Korean Patent Application No. 10-2003-0053873 and Korean Patent Application No. 10-2003-0018086 disclose use of sericin protein, and Journal of Cosmetic Dermatology, 2005, 4:250-257 mentions the possibility of using sericin protein as moisturizer. However, these articles only mention sericin and its decomposition product among silk protein, and the results are mostly for moisturizing effects limited for a short time, and thus, in practical use, sufficient moisturizing effect cannot be obtained.
And, Korean Registered Patent No. 10-0847299 discloses use of specific silk peptide fragments that have increased moisturizing effects by removing interference of fragments without moisturizing effects among the decomposition products of fibroin protein. However, according to the process of obtaining decomposition products, the yields and the moisturizing effects of the final products may be varied, and when applied for a cosmetic formulation, moisture retention time may not exceed 8 hours, and thus, there is a demand for performance improvement.